This invention relates to flame resistant latex compositions. More particularly, this invention relates to latex compositions comprising carboxylated styrene-butadiene-vinylidene chloride latices and a flame retardant additive, and to carpet and textile constructions coated, impregnated or laminated with these latex compositions.
Carboxylated styrene-butadiene latices find wide use in coating applications and as binders, adhesives, sizing, and backing in the construction of carpeting and floor coverings. The attractive properties of these materials, in particular the ease of compounding and rapidity with which they may be applied and processed, have contributed greatly to the wide acceptance of these systems. Additionally, these materials accept high levels of loading and may be formulated to emphasize abrasion resistance, insensitivity to moisture and biological attack, aging properties and adhesion in the finished structure. These latices may be made to be film forming at either ambient or elevated temperatures as the particular end use may require. Carboxylated styrene-butadiene polymers are inherently flammable, however, and are less than satisfactory for a great many commercial applications requiring flame resistant properties. Techniques for accomplishing the necessary reduction in flammability are well known, and the inclusion of chlorine or phosphorus containing compounds, often with a synergist such as antimony oxide, is widely practiced. Alternatively, high levels of inorganic fillers such as hydrated alumina have been incorporated in these latices to reduce susceptability to burning, often with the further addition of flame retardant compounds. In some instances, imparting flame resistance has been attempted by substituting a polyvinylchloride latex for a portion of the carboxylated styrene-butadiene latex, again normally with the inclusion of flame retardants and synergists in the composition. These approaches have not been completely satisfactory, however, in part because the flame retardant compounds employed are often expensive, and at the high levels often employed make the resulting latices considerably more costly to use. Further, many of these additions have a deleterious effect on the properties of the final product, for example, by decreasing flexibility and thereby increasing susceptability to cracking. Many of the flame retardant additives are subject to leaching and loss either during processing or subsequent use, and where high loading levels are employed, the ability to incorporate other necessary additives such as pigments, foaming agents and processing aids is severly restricted.
There is clearly a need for materials in latex form which can be formulated to give more highly flame resistant products without loss of the highly desirable processability and physical property features of carboxylated styrene-butadiene latices, and at a significant improvement in cost. To be of widest commercial acceptance, the base polymer employed should also be capable of sufficient structural variation to permit adjustment of such physical properties as hardness, flexibility and the like to meet the needs of a wide variety of end use applications.
It has now been found that latex compositions prepared by the emulsion polymerization of styrene, butadiene, vinylidene chloride and a suitable carboxylic acid monomer, are more resistant to flame in the finished form than conventional carboxylated styrene-butadiene polymers when compared by standard test methods and, when compounded with certain flame retardant additives, are markedly more resistant to flame. The latices of this invention exhibit the highly desirable processing and compounding characteristics attributed to conventional carboxylated styrene-butadiene latex systems, and can be varied in resulting physical properties from soft and rubbery to a less flexible character to suit particular end uses.
The particular polymers useful in the preparation of latices employed for purposes of this invention are those prepared by the emulsion polymerization of a monomer mixture containing from about 15% to about 45% by weight styrene, from about 25% to about 50% by weight butadiene, from about 25% to about 40% by weight vinylidene chloride and from about 1.0% to about 2.5% of a copolymerizable carboxylic acid.
The particular amounts of each monomer employed will depend somewhat upon the end-use envisioned. A minimum level of vinylidene chloride is necessary to impart good flame resistance to the resulting product, however, and it is essential that polymers contain greater than about 17% bound chlorine, corresponding to a 25% by weight level of vinylidene chloride incorporation, for good flame resistant characteristics. The more preferable compositions have incorporated at least 27% by weight vinylidene chloride, corresponding to 20% bound chlorine in the final polymer. Although further increases in vinylidene chloride level appear to impart a further flame retardant effect, the stiffness of the resulting product is also increased and at monomer levels above about 45% by weight the products are generally too non-resilient and stiff for most latex application.
The ratio of styrene and butadiene employed similarly affects hardness, rubbery character and tear strength of the resulting product, with low styrene, high butadiene compositions generally exhibiting a soft rubbery character while higher styrene systems are harder and more abrasion resistant. The precise levels to be employed are necessarily adjusted to account for the level of vinylidene chloride employed, and it has been found that at any level of styrene and vinylidene chloride, mixtures containing less than about 25% butadiene result in products too rigid and inflexible to be generally useful in latex applications, while products from compositions having greater than about 50% butadiene generally are too soft.
The carboxylic acid monomer may be any of the ethylenically unsaturated carboxylic acids commonly employed in the preparation of carboxylated styrene-butadiene latices, such as acrylic acid, methacrylic acid, cinnamic acid, crationic acid, itaconic acid, citraconic acid or maleic acid. Preferably, itaconic acid will be employed. The incorporation of small amounts, preferably from 0.5 to 10% by weight more preferably from 1.0 to about 2.5% by weight of a carboxylic acid in the monomer mixture imparts stability to the resulting latex and improves such product properties as adhesion and clarity.
The latices employed in the practice of this invention are preferably prepared by conventional emulsion polymerization methods although it is entirely conceivable that polymers of similar structure and composition might be prepared by other methods and subsequently converted to a latex form. Since the preferred emulsion process results directly in a suitable latex, it is therefore most practical and convenient.
The latices are futher compounded with a flame retardant additive. A great number of flame retardant additives are known to be useful in conventional carboxylated styrene-butadiene latex compositions and it is likely that most would impart at least some added flame resistance to the latices of this invention. It has now been found to be particularly advantageous to employ certain bromine compounds with carboxylated styrene-butadiene-vinylidene chloride latices. In particular, a bromine compound such as ammonium bromide or tris dibromopropyl phosphate, when incorporated into the latex at levels about 10 parts per hundred parts by weight of the latex on a dry solids basis, more preferably at a level above 30 parts per hundred and still more preferably at levels between about 30 parts by weight and about 60 parts by weight per hundred parts by weight of latex on a dry solids basis impart markedly improved flame resistance to finished products. These bromine compounds may be added to the latex upon completion of the polymerization step if water soluble, or during any later compounding step prior to use.